Flip chip die assembly using thin flexible substrates

ABSTRACT

Apparatus and methods for flattening thin substrate surfaces by stretching thin flexible substrates to which ICs can be bonded. Various embodiments beneficially maintain the substrate flatness during the assembly process through singulation. According to one embodiment, the use of a window frame type component carrier allows processing of thin laminates and flex films through various manufacturing processes. The flexible substrate is bonded to a rigid carrier. The carrier is placed into a specialized fixture comprising a bottom plate and a top plate. The bottom plate with raised regions is created that allows the windowed region of the flex film to be pressed flat. After aligning the top plate, the bottom plate, and the middle structure, the plates are pressed together causing the raised regions to push the flex film substrate upward and around the carrier. By pressing the thin substrate upward, the substrate is stretched like a drum head over the raised sections of the bottom plate, thereby flattening the substrate. The die assembly site is held flat overtop of the raised portion of the carrier to provide a stable vase for placement of the die.

TECHNICAL FIELD

Various embodiments of the present invention generally relate toapparatus and methods for assembling flip chip dies with thin flexiblesubstrates. More specifically, embodiments relate to apparatus andmethods flattening thin substrate surfaces by stretching thin flexiblesubstrates to which ICs can be bonded.

BACKGROUND

Traditionally, three approaches have been employed for connectingintegrated circuits (ICs) to printed circuit boards. These approachesare wire bonding, chip carriers with beam leads, and direct chipconnections. Flip chip technology is one of the direct chip connectionapproaches. In general, a flip chip assembly forms a direct electricalconnection between an electronic component and a substrate, circuitboard, or carrier, by means of conductive bumps on chip bond pads of theelectronic component.

Flip chip technology has been used since the early 1960's. Compared toother packaging methods, flip chip technology provides increasedperformance in several areas, such as smaller package sizes, increasedperformance in high frequency applications, improved thermalcapabilities, more I/O connections on a smaller die, and increasedreliability. With these advantages, flip chip technology has foundapplications in the computer industry, automotive applications,electronic watches, cellular phones, pagers, and the like.

There is a constant push in the industry to achieve smaller diepackages. Unfortunately, traditional flip chip assembly techniques havecharacteristics that limit the minimum thickness that can be achieved.One problem relates to the common use of the relatively thick, rigidpolymer substrates. With the current technologies, by bonding of thesilicon ICs to a rigid polymer substrate, devices currently incommercial production are on the order of 1 millimeter (mm) inthickness.

Flexible polymer substrates have been used in some approaches. However,previous direct chip connection approaches using thin flexible polymersubstrates have typically been unsuccessful. One problem relates touneven surfaces common to flexible polymer substrates. Such flexiblesubstrates do not have the proper flatness for accurately bonding ICsusing surface mount technologies (e.g., flip chip) with traditionalapproaches to die assembly. Because the polymers are so thin, it isdifficult to manufacture polymers having sufficiently flat surfaces.

Hence, there exists a need in the art for apparatus and methods forflattening the thin flexible substrates to facilitate accurate bondingof ICs with the substrate.

SUMMARY

Various embodiments of the present invention include apparatus andmethods for assembling flip chip dies including thin substrate surfaces.In some embodiments, the flip chip die is assembled by stretching thinflexible substrates to which ICs can be bonded. Stretching can beperformed using embodiments of a top plate and bottom plate, with theflexible substrate disposed therebetween. Compressing the flexiblesubstrate between the top plate and the bottom plate stretches portionsof the flexible substrate over the raised sections of the bottom plate.

In a particular embodiment of an apparatus, the top plate and bottomplate are positioned such that each raised section of the bottom plateis opposite an aperture of the top plate. The middle plate is positionedbetween the top and bottom plate in manner such that the carrierportions are above the network of channels created by the raisedsections of the bottom plate. The top plate is then pressed down on themiddle structure toward the bottom plate resulting in portions of thesubstrate to stretch over the raised sections of the bottom plate.

A more complete understanding may be derived by referring to thedetailed description of preferred embodiments and claims when consideredin connection with the figures.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. As will be realized, theinvention is capable of modifications in various obvious aspects, allwithout departing from the spirit and scope of the present invention.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures, similar components and/or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label with a second label thatdistinguishes among the similar components. If only the first referencelabel is used in the specification, the description is applicable to anyone of the similar components having the same first reference labelirrespective of the second reference label.

FIG. 1 a is a plan view of a top plate in accordance with one embodimentof the present invention;

FIG. 1 b illustrates the top plate of FIG. 1 a from another perspective;

FIG. 2 a is a plan view of a bottom plate in accordance with oneembodiment of the present invention;

FIG. 2 b illustrates the bottom plate of FIG. 2 a from anotherperspective;

FIG. 3 a is a cross-sectional view of a middle structure comprising aflexible substrate and a support frame in accordance with one embodimentof the present invention;

FIG. 3 b illustrates the middle structure of FIG. 3 a from anotherperspective;

FIG. 4 is a cross-sectional view of the top plate, middle plate, andbottom plate of the assembly fixture in accordance with one embodimentof the present invention;

FIG. 5 is a cross-sectional view of the assembled fixture stretching theflexible substrate in accordance with one embodiment of the presentinvention;

FIG. 6 is a cross-sectional perspective of the assembled fixture of FIG.5; and

FIG. 7 is a flow diagram illustrating a process of using the assemblyfixture in accordance with one embodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specific embodiments have been shown by way of example in thedrawings and are described in detail below. The intention, however, isnot to limit the invention to the particular embodiments described. Onthe contrary, the invention is intended to cover all modifications,equivalents, and alternatives falling within the scope of the inventionas defined by the appended claims.

DETAILED DESCRIPTION

Apparatus and methods are described for creating thin substrate surfacesby flattening thin flexible substrates to which ICs can be bonded.Embodiments presented below generally facilitate the manufacture ofthinner integrated circuits (ICs) by flatting thin flexible substratesto create a die assembly. ICs can be accurately bonded to the flattenedsubstrate using direct connection technologies, such as surface mount(e.g., flip chip).

According to one embodiment, the flip chip assembly process generallycomprises one or more of the following: (1) fabrication of the componentor chip substrate; (2) fabrication of a bumped IC wafer; (3) singulationof the die from the wafer; (4) picking and inspection of die from a diefeeder; (5) fluxing the die or carrier site by dipping, transfer, spray,dispensing, or brushing methods; (6) placement of the die onto thefluxed site; (7) fusing of the solder bumps to the attachment pads byreflow soldering; (8) post reflow underfill (e.g., epoxy dispensing);(9) epoxy cure; (10) solder bump attachment; (11) singulation (i.e., thecutting of a wafer into individual die); and (12) test and finalprocessing.

In general, there are several steps typically performed to fabricate asemiconductor component. At a high level, various embodiments relate tothe interconnection of the terminations on the IC with the remainingdevices on the circuit board. Typically, the IC termination pads arevery small and the spacing (pitch) between conducting pads is equallysmall. This makes direct connection to the circuit board morechallenging. In addition, testing and burn in of the IC is difficult inthe raw IC format and thus requires the die to be packaged in a housingof reasonable size etc. to allow testing in sockets or probe stations.Therefore, a component package is useful not only to house the IC but toallow a more robust test configuration and a larger termination pitchfor easy assembly.

There are several methods to interconnect the IC with the circuit board.One way is to package the IC in an organic container (component body).The component will house the IC, protect it and allow for proper testingetc. The first level connection (between the IC and component) can beaccomplished in several ways including wirebond, ribbon bond, conductiveadhesives and soldering. In the soldering case, for example, the IC hassolderable termination pads, onto which solder is deposited at the ICfabricator. To join the IC solder bumps with corresponding attachmentpads on a conductive layer in our component a flexible film with coppercircuits and attachment pads in our prototypes may be used. The flexfilm has a circuit pattern that allows the IC bumps to align with thecomponent film pads.

Flux is most often utilized to clean the attachment pads and the ICbumps prior to solder joining. The flux is a resin most often made ofpine tree sap (acidic) that when heated removes all oxides on the metalconductive surfaces and allows for a proper melting of the solder tooccur. The flux is deposited onto the attachment pads on the flex filmdirectly (for example fluid dispense, pin transfer, or jetting) or itcan be applied the bumps on the IC by dipping the IC into a flux film. Asolder paste (flux plus solder particles) may also be used to join theIC pads and circuit pads on the component. Once the flux or paste isapplied, the IC is brought into contact with the component by alignmentof the IC bumps and component pads in a placement machine.

The reflow process generally involves heating the assembly in acontrolled profile (time and temperature) until the flux activates,oxides are removed, and the solder is melted. Once melted, the solderjoins with the attachment pad surface and makes a metallurgical bond.Upon cooling, the solder solidifies and the solder interconnection iscomplete. Often the IC is further protected by deposition of anencapsulating material either over or in between the IC and componentsurface. The encapsulation process can be achieved by dispensing orjetting a liquid epoxy or by over-mold injection. Once the resin iscured, it protects the IC and component's fragile circuitry.

In addition to the primary IC, a component may have other ICs orcomponents such as de-coupling capacitors that are required to completethe circuit. These components are typically assembled onto the componentsubstrate prior to any molding or encapsulation process and typicallyutilize a solder paste process described above.

Once the component has the IC and subsequent components attached to thesubstrate, other features may be required to complete the functional oraesthetic aspects of the device. For example, once finished, thecomponent will be ultimately used in building an electronic device orapparatus of some kind. Typically solderable attachment pads or bumpsare provided on the component to attach the component to another circuitboard. These are often applied after the IC is bonded as describedabove. Therefore, solder bumps or other termination processing may occurafter the IC is assembled. In addition, marking of the component toidentify device number, part number, etc. may be required. Those stepsare often performed prior to removing or singulating the component fromthe flex film strip. Finally, individual components are generallysingulated into single units.

Embodiments described herein beneficially maintain the substrateflatness during the assembly process through singulation. According toone embodiment, the use of a window frame type component carrier allowscircuit board manufacturers to process thin laminates and flex filmsthrough their manufacturing processes. The flex film is bonded to arigid carrier. The carrier is placed into a specialized fixture. Abottom pallet with raised regions is created that allows the windowedregion of the flex film to be pressed flat. A top portion of the carrieris joined to the bottom part of the carrier such that the windowedregion of the flex film is stretched (e.g., like a drum or knittinghoop). The die assembly site is held flat overtop of the raised portionof the carrier to provide a stable vase for placement of the die. Thecarrier retains the flex board stability during subsequent reflowprocesses.

Terminology

Brief definitions of terms and/or phrases used throughout thisapplication are given below. The terms “connected” or “coupled” andrelated terms are used in an operational sense and are not necessarilylimited to a direct connection or coupling. Thus, for example, twodevices may be coupled directly, or via one or more devices. Based onthe disclosure provided herein, one of ordinary skill in the art willappreciate a variety of ways in which connection or coupling exists inaccordance with the aforementioned definition.

The phrases “in one embodiment,” “according to one embodiment,” and thelike generally means the particular feature, structure, orcharacteristic following the phrase is included in at least oneembodiment of the present invention, and may be included in more thanone embodiment of the present invention. Importantly, such phrases donot necessarily refer to the same embodiment.

If the specification states a component or feature “may,” “can,”“could,” or “might” be included or have a characteristic, property, orstructure, that particular component or feature is not required to beincluded or have the characteristic, property, or structure.

According to some embodiments of the invention, a thin polymer substrateis first bonded to a carrier plate. The thin polymer substrate can be apolymer, such as, but not limited to, Kapton-polyimide, liquid crystalpolymer (LCP), or EPTFE and can be about 0.0005 to 0.005 inches thick.The carrier is typically a rigid polymer substrate, such as FR4, and isabout 0.015 to 0.125 inches thick. To flatten out the thin polymer, apress can be used, which includes a top plate and a bottom plate.

The top plate and the bottom plate of the press are generally formedfrom material which has a low thermal mass. In addition, the materialfrom which the top plate 100 and bottom plate 200 are formed from shouldbe light and capable of surviving processing temperatures on the orderof 300 degrees Celsius. Thus, as just some examples, materials fromwhich the top plate 100 and bottom plate 200 can made primarily ofinclude, but are not limited to, aluminum, titanium and Delmat.

Referring now to FIGS. 1 a–1 b, one embodiment of top plate 100 inaccordance with the present invention is shown. In the illustratedembodiment, top plate 100 comprises a plurality of apertures 110. Theopenings 110 in the top plate allow for the placement of the ICs andother components for bonding to the substrate. In the embodimentdepicted, the apertures 110 form rectangles which are aligned verticallyand horizontally. In other embodiments, the shape of the apertures neednot be limited to rectangles. Other examples of aperture shapes includeany other geometric shape or custom design. Furthermore, each aperture110 on the top plate need not be the same shape. In yet otherembodiments, the apertures 110 are not aligned vertically andhorizontally.

One embodiment of the bottom plate 200 is depicted in FIGS. 2 a–2 b. Thebottom plate includes raised regions 210. The raised regions 210 arepositioned such that each raised section is aligned with an opening 110of the top plate. Between the raised regions 210 a network of channels230 is formed. In the illustrated embodiment, the raised sections 210are rectangles which are aligned vertically and horizontally. The shapeof the raised section 210, however, is not limited to rectangles alignedvertically and horizontally. Typically, the shape and placement of theraised sections 210 should be similar in design and placement as theopenings 110 in the top plate.

FIGS. 3 a–3 b provide views of one embodiment of the middle structure300. A thin flexible substrate 310 is mounted to a carrier, or supportframe, 320. The carrier 320 forms a raised section capable of meshingwith the network of channels 230 located on the bottom plate 200. Thecircuit pattern or design on the flexible substrate depends on the ICdesign. In this particular embodiment, the thickness of the flexiblesubstrate 310 ranges from 0.0005 to 0.005 inches and may containmultiple layers depending on the design. Examples of primary substratematerials include, but are not limited to, polyimide, liquid crystalpolymer and polytetraflouroethylene (EPTFE). The carrier 320 typicallyhas a thickness in the range of 0.015 to 0.125 inches. The carrier istypically, but not necessarily, composed of a cheaper material such as areinforced glass laminate (e.g. FR4).

FIG. 4 illustrates how the top plate 100, bottom plate 200, and themiddle structure 300 can be aligned in accordance with one embodiment.The top plate 100, bottom plate 200, and middle structure 300 arepositioned substantially parallel to each other, with the middlestructure between the top plate 100 and bottom plate 200. The openings110 of the top plate 100 are aligned overtop of the raised portions 210of the bottom plate 200. The support frame 320 is aligned with thenetwork of channels 230 of the bottom plate 200, such that when the topplate 100 is pressed down toward the bottom plate 200, the support frame320 meshes with the network of channels 230.

In accordance with the particular embodiment illustrated in FIGS. 5–6,the flex film substrate and carrier are sandwiched between to top plate100 and bottom plate 200. By pressing the plates together, the raisedregions of the bottom plate 200 push the flex film substrate upward andaround the carrier. By pressing the thin substrate upward, the substrateis stretched like a drum head, thereby flattening the substrate 310. Inthe embodiments depicted in FIGS. 1–4, each plate has screw holes, 120,220, and 330, to aid in alignment of the structures and to helpfacilitate compressing the middle structure 300 between the top plate100 and the bottom plate 200. In other embodiments, other mechanisms andstructures (e.g. clamps) may be used for the alignment and sandwichingof the plates.

Having described an exemplary assembly apparatus, a block diagram 700representing a high level overview of the use according to oneembodiment of the present invention will now be described with referenceto FIG. 7. At step 710, the middle fixture and substrate are placed overthe bottom plate in such a manner that the network of channels in thebottom plate are aligned with the support frame of the middle structure.On top of the middle structure the top plate is aligned in a manner thatthe openings of the top plate are positioned over the raised sections ofthe bottom plate at step 720. The substrate is compressed between thetop and bottom plate at step 730 causing the substrate to stretch acrossthe raised portions of the bottom plate.

After the flexible substrate is stretched, a die can be assembled bybonding one or more ICs to the flattened substrate through openings inthe top carrier using flip chip technologies. In some embodiments, afterthe ICs are bonded to the thin substrate, the top plate, and the bottomplate are removed and the substrate and IC combination are cut away fromthe carrier by mechanical separation or through laser cutting process.Thus, all that remains is the thin substrate (<0.001 inches), theintegrated circuit (about 0.008 inches), and the solder used to bond thetwo together (<0.003 inches). The resulting packaged IC can be thinnerthan an IC constructed with conventional approaches. Additionally, othercomponents such as de-coupling capacitors, small resistors, or other ICsmay be bonded in close proximity to the primary IC die to create amulti-chip device or module.

In conclusion, embodiments relate to novel apparatus, methods andarrangements for assembling an IC die by flattening a flexiblesubstrate. While detailed descriptions of one or more embodiments of theinvention have been given above, various alternatives, modifications,and equivalents will be apparent to those skilled in the art withoutvarying from the spirit of the invention. Therefore, the abovedescription should not be taken as limiting the scope of the invention,which is defined by the appended claims.

1. A method for creating a flat substrate surface, the methodcomprising: aligning a flexible substrate between a frame and a bottomplate, wherein the bottom plate includes one or more raised sectionshaving channels therebetween, and wherein the frame includes openingsfacing the raised sections of the bottom plate; and compressing theflexible substrate between the frame and the bottom plate, such that theopenings of the frame mesh with the raised sections of the bottom plate,thereby stretching one or more portions of the flexible substrate acrossthe raised sections of the bottom plate.
 2. The method of claim 1,wherein compressing comprises: aligning a top plate above the frame, thetop plate having one or more openings aligned with the openings of theframe; and pressing the top plate down upon the frame forming asubstrate assembly.
 3. The method of claim 1 further comprising:depositing flux or a conductive media on the flexible substrate throughthe one or more openings of the top plate; placing one or more componentdevices on the flexible substrate of the substrate assembly through oneor more associated openings of the top plate; attaching component deviceterminations to stretched portions of the flexible substrate usingsolder; and heating the substrate assembly with attached componentdevices to form one or more assembled components.
 4. The method of claim3, further comprising separating the one or more assembled components.5. The method of claim 3, wherein the conductive media is a solderpaste.
 6. The method of claim 3, wherein the device terminations aresubsequent solder bumps.
 7. The method of claim 3, further comprising:the deposition of an underfill or overmold compounds; and curing of saidunderfill or overmold compounds.